The invention relates generally to multiple compressor refrigeration systems, and more particularly to improvements in suction pressure controls for the compressors.
In recent years many advances have been made in the refrigeration art and especially in the commercial refrigeration field, which includes supermarket refrigeration and like installations having heavy refrigeration requirements over a wide range of temperatures from about -40.degree. F. to about 50.degree. F. So-called central refrigeration systems of the heavy multiplexing type utilize several compressors (typically either two or four) connected for parallel operation to effect refrigerant flow to and from the evaporators of a large number of refrigerated fixtures. Multiple compressor systems are generally controlled by pressure sensitive switches responsive to the suction pressure at the compressors intake so that as the suction pressure fluctuates in response to increases or decreases in system loads, the compressors will cycle on and off to maintain the common suction pressure on the system within prescribed limits as required to maintain proper temperature control of the refrigerated fixtures.
Fluctuation of the suction pressure is influenced by various internal (system) factors including temperature controls, defrosting apparatus and the like, and by several external factors including product loading of refrigerated fixtures, ambient temperatures and the like, and at times sudden transient increases in suction pressure may cause one or more idle compressors to start thereby rapidly reducing the suction pressure to the point where such compressors will cycle off again. Since these suction pressure changes are frequently transient in nature, the capacity of the operating compressors in the system would often be adequate to restore the normal suction pressure operating the system before there is any significant influence on the refrigerated compartment temperatures of the fixtures. However, if the thermal load change causing the suction pressure (temperature) rise is of long duration, then the operation of one or more additional compressors may be necessary to maintain normal refrigerated compartment temperatures.
It is apparent that electric power consumption will be reduced in the overall operation of the refrigeration system if additional compressors are not started in response to transient load increases. In the past, electric time delay relays have been used for delaying the start of additional compressors sensing an increase in suction pressure, but such electric relays are insensitive to the actual magnitude of suction pressure, whereby the compressor controlled thereby will start no matter how small the difference between actual suction pressure and the pressure switch setting of the compressor. In short, heretofore there has been no simple, positive acting, suction pressure control for effectively obviating on/off compressor cycling due to sudden temporary or transient suction pressure changes.